Carbyne Ring Activated Using ZnCl₂ for Hydrogen Adsorption: DFT Study

We have studied the feasibility of activated carbyne as a good hydrogen storage material. Density functional theory (DFT) simulations through van der Waals interactions have been applied to investigate calcium sorption on activating carbyne with zinc dichloride (ZnCl₂) and also interactions of molecular hydrogen with pristine carbyne and Ca functionalized on an activated carbyne C₁₂-ring. The obtained results showed that (i) the chemical activation of the C₁₂-ring with ZnCl₂ increases its area by 5.17% with respect to pristine carbyne. (ii) Ca atoms at small concentrations tend to get atomically sparse on carbyne, donating +0.94e and +1.05e to the ring, according to Mulliken population analysis and the electrostatic potential fitting charges, respectively. Furthermore, in the presence of calcium, hydrogen sorption increases by 21.8% in comparison with Ca-decorated pure carbyne. (iii) Seven hydrogen molecules per Ca atom have adsorption energy close to the range of ∼0.3-0.5 eV per H₂, which is necessary for effective charge/discharge cycles. (iv) Theoretical uptake (7.11 wt %) with a single Ca atom is higher than the U.S. Department of Energy target (5.5 wt %). Therefore, an activated C₁₂-ring can bind three Ca atoms with its seven H₂ molecules reaching 13.8 wt %. (v) Equilibrium pressure for CaC₁₂-7H₂ and Ca₃C₁₂-21H₂ systems (5-15 MPa) by means of adsorption isotherm calculations. The calculated van't Hoff desorption temperatures exceed considerably the boiling point of liquid nitrogen. In addition, we also performed DFT-based molecular dynamics simulations for the C₁₂, CaC₁₂, CaC₁₂-7H₂, and Ca₃C₁₂-21H₂ systems to study thermal stability. Our results confirm the potential of Ca-decorated carbyne for hydrogen storage.